Method for checking the plausibility of the measured load in an internal combustion engine having variable valve lift control

ABSTRACT

The plausibility check takes place in such a way that a first piece of load information is derived as a function of the throttle valve position and a differential pressure determined from the pressure upstream of the throttle valve and the pressure downstream of the throttle valve. In addition, a second piece of load information is derived as a function of the intake valve lift and the engine speed. The smaller of the two pieces of load information is compared with the measured load. If this measured load deviates from the load information an error of the measured load is signaled.

FIELD OF THE INVENTION

The present invention relates to a method for checking the plausibilityof the measured load in an internal combustion engine having variablevalve lift control.

BACKGROUND INFORMATION

Such a plausibility check is, for example, practical with torquemonitoring in which an acceptable engine torque, which essentiallyresults from the engine speed and the accelerator pedal position, iscompared with an actual torque calculated from engine variables. As arule, these engine variables include the ignition angle, the enginespeed and the engine load. In order for the effectiveness of the torquemonitoring not to be limited by continuously too little loadinformation, the plausibility of the measured load signal should bechecked with regard to an excessively low value. A comparison of loadinformation obtained from the throttle valve position and the enginespeed with the measured load is known for a conventional internalcombustion engine from European Published Patent Application No. 0 778406. If the measured load varies from the load information derived thethrottle valve position and the engine speed by a specific amount, anincorrect load measurement is assumed and an error correction isinitiated accordingly.

SUMMARY OF THE INVENTION

An object of the present invention is now to specify a method of theaforementioned type with which a measured load can also be checked forplausibility in an internal combustion engine having variable valve liftcontrol.

The stated objective is attained in that an initial piece of loadinformation is derived as a function of the throttle valve position anda differential pressure determined from the pressure upstream of thethrottle valve and the pressure downstream of the throttle valve in theintake manifold. A second piece of load information is derived as afunction of the intake valve lift and the engine speed. The smaller ofthe two pieces of load information is finally compared with the measuredload and an error of the measured load is signaled if it deviates fromthe load information with which it is compared. With this methodaccording to the present invention, it is taken into account that inaddition to the throttle valve, the valve lift control also brings abouta throttling effect.

For the event that the determined differential pressure or the intakevalve lift is incorrect and thus no throttling is possible via thevalves, it is advantageous to derive a third piece of load informationas a function of the throttle valve position and the engine speed andthen to make a comparison with the third piece of load information for aplausibility check of the measured load.

For a very reliable plausibility check of the measured load, it isadvantageous to also check the determined differential pressure and theintake valve lift for plausibility. The plausibility check of thedifferential pressure is performed in such a way that differentialpressure information is derived as a function of the throttle valveposition and the engine speed, this information being compared with thedetermined differential pressure, and an error of the differentialpressure is signaled if the determined differential pressure is lessthan the differential pressure information. The plausibility check ofthe intake valve lift can be performed in such a way that the secondpiece of load information derived from the intake valve lift and theengine speed is compared with the measured load or a load derived fromthe throttle valve position and the intake manifold differentialpressure, and an error of the intake valve lift is signaled if thesecond piece of load information is less than the measured load or aload derived from the throttle valve position and the intake manifolddifferential pressure.

The first piece of load information is advantageously obtained in such away that the air mass flow to the throttle valve is read out of anengine characteristics map as a function of the throttle valve positionand the differential pressure and that this air mass flow is divided bythe engine speed which is multiplied by an engine-specific factor whichis a function of the number of cylinders and the piston displacement.

The second piece of load information is advantageously obtained in sucha way the air mass flow through the intake valve(s) is read out of anengine characteristics map as a function of the intake valve lift andthe engine speed and that this air mass flow is divided by the enginespeed which is multiplied by an engine-specific factor which is afunction of the number of cylinders and the piston displacement.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a system representation of the pressures and air massflows.

FIG. 2 shows a function diagram for the plausibility check of themeasured load.

FIG. 3 shows a function diagram for the plausibility check of themeasured differential pressure.

FIG. 4 shows a function diagram for the plausibility check of the inletvalve lift.

DETAILED DESCRIPTION

FIG. 1 shows a system representation of the pressures and air mass flowsin the intake manifold of an internal combustion engine in which an airmass meter (preferably a hot film air mass meter) HFM for loaddetection, a throttle valve DK and at least one intake valve VV, thelift of which is variably controllable are located. The air mass flowupstream of air mass flow meter HFM is identified as mhfm, the air massflow upstream of throttle valve DK as mdk, the air mass flow through theat least one intake valve VV as mvv and the air mass flow to thecombustion chamber of the engine as mbr. The pressure upstream ofthrottle valve DK is identified as pvdk, the intake manifold pressuredownstream of throttle valve DK as ps and the combustion chamberpressure as pbr. The following relationships may be stated with regardto this system representation of the pressures and air mass flows:

In general, as shown in equation (1), in a steady state, the air massflows are of equal magnitude through air mass meter HFM, throttle valveDK and through the at least one intake valve VV.

mhfm=mdk=mvv=mbr  (1)

Equation (2) describes air mass flow mdk through throttle valve DK. Thisair mass flow mdk results from a standard air mass flow mdkn as afunction of throttle valve position dk multiplied by a pressurecorrection factor, a temperature correction factor, and a factor from athrottle valve flow characteristic KLAFDK. $\begin{matrix}{{mdk} = {{{mdkn}({dk})} \cdot \frac{pvdk}{p0} \cdot \sqrt{\frac{T0}{Tvdk}} \cdot {{KLAFDK}\left( \frac{ps}{pvdk} \right)}}} & (2)\end{matrix}$

Standard air mass flow mdkn is the air mass flow through the throttlevalve at a defined throttle valve position, a defined temperature T0,and a defined pressure p0 upstream of the throttle valve. The pressurecorrection factor is the ratio of pressure pvdk upstream of throttlevalve DK to pressure p0. The temperature correction factor is the squareroot of the ratio of temperature T0 to temperature Tvdk upstream of thethrottle valve. Factor KLAFDK originates from a throttle valve flowcharacteristic as a function of the ratio of intake manifold pressure psdownstream of the throttle valve to pressure pvdk upstream of thethrottle valve.

Disregarding pressure pvdk upstream of the throttle valve, air mass flowmdk through the throttle valve can also be described as a function ofthe throttle valve position and of the intake manifold differentialpressure.

The air mass flow through the intake valves is described in equation(3). $\begin{matrix}{{mvv} = {{{mvvn}({vv})} \cdot \frac{p\quad s}{p0} \cdot \sqrt{\frac{T0}{Ts}} \cdot {{KLAFVV}\left( \frac{pbr}{p\quad s} \right)}}} & (3)\end{matrix}$

Air mass flow mvv through the valves is the product of a standard airmass flow mvvn through the valves as a function of valve position vvmultiplied by a pressure correction factor, a temperature correctionfactor, and a factor of a valve outlet characteristic KLAFVV. Standardair mass flow mvvn through the valves corresponds to the air mass flowat a defined pressure p0 and a defined temperature T0. The pressurecorrection factor is the ratio of intake manifold pressure ps downstreamof throttle vale DK to pressure p0. The temperature correction factor isthe square root of the ratio of temperature T0 to temperature Ts in theintake manifold downstream of throttle valve DK. Factor KLAFVVcorresponds to the value of the valve outlet characteristic as afunction of the ratio of combustion chamber pressure pbr to intakemanifold pressure ps. In first approximation, the valve outletcharacteristic can be seen as a measure for the flow velocity of the gasflowing through the at least one intake valve.

Disregarding intake manifold pressure ps, air mass flow mvv through atleast one valve can also be regarded as a function of the valve lift andof the engine speed.

Differential pressure dps in the intake manifold is shown in Equation 4,the differential pressure indicating the difference between pressurepvdk upstream of throttle valve DK and pressure ps downstream of thethrottle valve. Or expressed in other terms, intake manifolddifferential pressure dps is equal to the difference between ambientpressure pu and intake manifold pressure ps downstream of throttle valveDK.

dps=pvdk−ps=pu−ps  (4)

As the function diagram in FIG. 2 shows, air mass flow mdk to thethrottle valve is obtained from a first engine characteristics map KF1as a function of throttle valve position dk and differential pressuredps. Throttle valve position dk is customarily detected by way of twopotentiometers with the result that a plausibility check of throttlevalve signal dk is possible. Differential pressure dps arises bymeasurement of the two pressures pvdk upstream of throttle valve DK andps downstream of throttle valve DK. The air mass flow read out of thefirst engine characteristics map KF1 is led across a low-pass filter FIto filter specific dynamic components out of the air mass flow signalwhich arise with the change of the throttle valve position. Moreover,air mass flow mvv through the at least one intake valve is read out of asecond engine characteristics map KF2 as a function of intake valve liftvv and engine speed nmot.

A minimum value MIN of the two air mass flows is formed and the smallerof the two air mass flows mdk, mvv is supplied to a divider DIV1. Fromthe smaller of the two air mass flows mdk, mvv and engine speed nmot,divider DIV1 forms a value which is multiplied in a multiplier MU1 by anengine-specific constant KUMSRL. This engine-specific constant KUMSRL isessentially a function of the number of cylinders and the pistondisplacement of the engine. The output signal of divider DIV1 thenrepresents a piece of load information 11 or 12. Depending on whetherthe throttle valve or the at least one intake valve has a greaterthrottling effect, either a first piece of load information 11 as afunction of throttle valve position DK or a second piece of loadinformation 12 as a function of intake valve lift vv is available at theoutput of divider DIV1.

The first or second piece of load information 11 or 12 is supplied via aswitch SW to a threshold value discriminator SE1 if switch SW hasassumed its switching position 1. Switch SW is in switching position 1if neither of the two conditions BVV and BDS present at the inputs of anOR gate OD is met. If condition BVV has the value 1, this means that theat least one intake valve is set to maximum lift; the variable valvecontrol is thus not active and the engine is therefore in conventionaloperation. If condition BDS has the value 1, this indicates that apressure sensor error is present and therefore differential pressure dpsis incorrect; thus throttling via the valves is no longer possible.Switch SW is thus in switching position 1 and threshold valuediscriminator SE1 receives either the first or the second piece of loadinformation 11, 12 as an input signal if both conditions BVV and BDShave the value 0, which means that both the variable valve control isfunctioning properly and the pressure sensor or pressure sensors aremeasuring a correct differential pressure dps.

Conditions BVV and BDS are derived irrespective of information vv anddps. Information, e.g., feedback from the valve control unit, electricalplausibility checking of the pressure signal, which need not beprotected for proper function, is used to generate these conditions BVVand BDS. In the event of an unauthorized setting of condition BVV or BDSto logical 1, throttling via the valves is stopped and conventionalerror monitoring as described in European Published Patent ApplicationNo. 0 778 406 cited above can be performed. If condition BVV or BDS iserroneously not set to logical 0, this is recognized with theplausibility check applied here.

If one of the two conditions BVV or BDS now has the value 1 as theresult of an error, the output signal of OR gate OD of switch SW ismoved into switching position 2. In this case, a third piece of loadinformation 13 is switched through to threshold value discriminator SE1via switch SW. The third piece of load information 13 is read out of athird engine characteristics map KF3 as a function of throttle valveposition dk and engine speed nmot. Depending on whether the three piecesof load information 11, 12 or 13 are present at threshold valuediscriminator SE1, it compares the relevant piece of load information11, 12 or 13 with the load lhfm measured by air mass sensor HFM. Ifmeasured load lhfm falls below load information 11 or 12 or 13, thenthreshold value discriminator SE1 signals an error fhfm of the measuredload. Threshold value discriminator SE1 allows, however, a certaintolerance in the deviation of measured load signal lhfm from loadinformation 11, 12 or 13 before it outputs an error signal fhfm.

To be able to perform a correct plausibility check for the measured loadthe information concerning throttle valve position dk, the informationconcerning differential pressure dps, and the information concerningvalve lift vv are also to be free of errors.

A plausibility check of throttle valve position dk takes place in aknown manner by using two potentiometers which measure the throttlevalve position. This plausibility check will therefore not be explainedin greater detail.

The function diagram shown in FIG. 3 depicts a plausibility check fordifferential pressure dps. In this connection, a threshold valuediscriminator SE2 compares measured differential pressure dps with adifferential pressure dpi obtained from an engine characteristics mapKF4 as a function of throttle valve position dk and engine speed nmot.Low-pass filter Fi connected downstream of engine characteristics mapKF4 has the same function as low-pass filter Fi shown in FIG. 2, namelyto suppress undesirable signal components caused by changes in thethrottle valve position. If threshold value discriminator SE2 detectsthat measured differential pressure dps is greater than differentialpressure information dpi, it then signals an error of measureddifferential pressure dps by outputting a logical 1 to an AND gate A1.An error signal fdps in the form of a logical 1 appears at the output ofthis AND gate A1 only if two conditions BDS and BDK are also fulfilledsimultaneously. Condition BDS has the value 1 if the differentialpressure sensor is in proper order and condition BDK has the value 1 ifthe sensor system operates properly in detecting the throttle valveposition. Only if both of these two conditions BDS and BDK are met, doesthe described plausibility check make sense for differential pressuredps and AND gate A1 should output an error signal fdps only if measureddifferential pressure dps deviates from differential pressure dpiderived from engine characteristics map KF4 by a specific degree.

The explanation already given in connection with FIG. 2 in relation toconditions BVV and BDS also applies to the two conditions BDS and BDKand their independence from information dps and dk. Electrical signalsfor checking the plausibility of the intake manifold differentialpressure and the throttle valve position can be used to deriveconditions BDS and BDK.

The function diagram shown in FIG. 4 illustrates the plausibility checkof intake valve lift vv. For this purpose, the second piece of loadinformation 12 derived from intake valve lift vv and engine speed nmotis compared with measured load lhfm by a threshold value discriminatorSE3. Load information 12 derived from intake valve lift vv and enginespeed nmot is obtained in the same manner as already described in thefunction diagram of FIG. 2 in that as a function of intake valve lift vvand engine speed nmot, air mass flow mvv through the at least one intakevalve is obtained from engine characteristics map KF2 and this air massflow mvv is divided by divider DIV2 through engine speed nmot to whichan engine-specific constant (as a function of number of cylinders,piston displacement) is applied by multiplier MU2. Load information 12is available at the output of divider DIV2. Before load information 12at the output of divider DIV2 is supplied to threshold valuediscriminator SE3, it is increased by an additional tolerance amount TOLby way of an adder AD. Threshold value discriminator SE3 supplies alogical 1 at its output if measured load signal lhfm is greater thanload information 12 derived from intake valve lift vv and engine speednmot. In this case, the intake valve lift is in fact incorrect. Insteadof measured load lhfm, a load derived from the throttle valve positionand the intake manifold differential pressure may be compared with loadinformation 12.

An AND gate A2 emits a logical 1 from threshold value discriminator SE3signaling an error as an error signal fvv at its output only if the twoconditions BVV and BDS appear simultaneously with a logical 1 at theinputs of AND gate A2 simultaneously. Condition BVV states that, if ithas logical condition 1, the valve lift information is in proper order.Condition BDS indicates that, if it is in logical condition 1, thedifferential pressure sensor is functioning properly.

What is claimed is:
 1. A method for checking a plausibility of ameasured load in an internal combustion engine including a variablevalve lift control, comprising the steps of: deriving a first piece ofload information as a function of a throttle valve position and adifferential pressure determined from a pressure upstream of a throttlevalve and a pressure downstream of the throttle valve in an intakemanifold; deriving a second piece of load information as a function ofan intake valve lift and an engine speed; selecting a smaller one of thefirst piece of information and the second piece of information;comparing the selected smaller one of the first piece of information andthe second piece of information with the measured load; and signaling anerror in the measured load if the measured load deviates from theselected smaller one of the first piece of load information and thesecond piece of load information compared thereto.
 2. The methodaccording to claim 1, further comprising the steps of: deriving a thirdpiece of load information as a function of the throttle valve positionand the engine speed; and performing a plausibility check of themeasured load by performing a comparison with the third piece of loadinformation only if one of the determined differential pressure and theintake valve lift is incorrect.
 3. The method according to claim 1,further comprising the steps of: performing a plausibility check of thedetermined differential pressure by deriving a piece of differentialpressure information as a function of the throttle valve position andthe engine speed; comparing the piece of differential pressureinformation with the determined differential pressure; and signaling anerror in the determined differential pressure if the determineddifferential pressure is less than the differential pressureinformation.
 4. The method according to claim 1, further comprising thesteps of: performing a plausibility check of the intake valve lift bycomparing the second piece of load information with one of the measuredload and a load derived from the throttle valve position and an intakemanifold differential pressure; and signaling an error in the intakevalve lift if the second piece of load information is less than one ofthe measured load and the load derived from the throttle valve positionand the intake manifold differential pressure.
 5. The method accordingto claim 1, wherein the first piece of load information is obtained byperforming the steps of: reading an air mass flow to the throttle valveout of an engine characteristics map as a function of the throttle valveposition and the differential pressure, and dividing the air mass flowby the engine speed, the engine speed being multiplied by anengine-specific constant that is a function of a number of cylinders anda piston displacement.
 6. The method according to claim 1, wherein thesecond piece of information is obtained by performing the steps of:reading an air mass flow through an intake valve out of an enginecharacteristics map as a function of the intake valve lift and theengine speed, and dividing the air mass flow by the engine speed, theengine speed being multiplied by an engine-specific constant that is afunction of a number of cylinders and a piston displacement.